As gasoline or other fuel is pumped into an automobile or other motor vehicle from a fuel delivery system, fuel vapor is released from the receiving tank. These vapors must be collected to prevent their escape and pollution of the surrounding environment. Vapor recovery systems are currently used to collect vapors released during a fueling operation. A current product of Gilbarco, Inc., assignee of the present invention, sold under the name VaporVac.RTM. collects vapor released during a fueling operation by using a vapor pump to pump vapors into the vapor recovery system. The rate at which vapor is collected is controlled by varying the speed of the vapor pump. For maximum performance and efficiency of a vapor recovery system, the speed of the vapor pump must be controlled to collect vapor at a rate that corresponds to the instantaneous vapor volume released or generated during a fueling operation while drawing in little or no air.
As is pointed out in U.S. Pat. No. 5,040,577 to Pope, U.S. Pat. No. 5,156,199 to Hartsell et al. and co-pending U.S. patent application Ser. No. 07/968,595, now U.S. Pat. No. 5,345,979 filed Oct. 29, 1992, the rate at which the vapor must be recovered is determined by several variables including the liquid fuel flow rate, the liquid fuel temperature, the ambient temperature and the amount of fuel dispensed in the current fueling operation.
To operate the vapor pump at an optimal speed, the vapor volume generated is continuously determined by a processor during a fueling operation. The processor computes the instantaneous vapor volume generated and produces corresponding vapor pump control signals that are sent to the vapor pump. The control signals adjust the speed of the vapor pump so that the rate of vapor recovery corresponds to the computed vapor volume generated.
The processor generates the control signal to be sent to the vapor pump by solving a control function. In known vapor recovery systems, the solution to the control function is a value related to the ratio of the instantaneous volume of vapor generated divided by the instantaneous volume of liquid fuel (V/L) dispensed. during a fueling operation. The vapor recovery system uses the derived V/L ratio to generate the control signal for controlling the speed of the vapor pump such that the rate at which released fuel vapor is collected is as close as possible to the rate at which vapor is generated during a fueling operation.
As mentioned, the control function used to generate the vapor pump control signal is dependent on a plurality of independent variables which each affect the instantaneous volume of fuel vapor generated during a fueling operation. The independent variables of the control function include flow rate, volume dispensed, time, ambient temperature, fuel temperature, and restrictions in the vapor path. The control function is solved by measuring the independent variables and inputting the measured values into the control function.
To precisely determine the optimal vapor pump speed, a complex control function that models or approximates the thermodynamic, fluid, gas, and other physical laws which ultimately govern the V/L ratio must be solved. Such a complex control function takes into account a plurality of independent variables and requires intensive numerical operations. Implementation of a vapor recovery system that relies on a complex control function to determine optimal vapor pump speed would require a moderate or high-speed processor. Examples of control functions of this sort are shown in the Haansell et al. patent, supra and in U.S. Pat. No. 5,038,838 to Bergamini et al.
A moderate or high speed processor is required because the processor must be sufficiently proficient to determine the solution to the control function in a time period that does not unduly degrade the accuracy of the system. If an extended period of time is required, the phase margin of the system will be substantially degraded. That is, by the time the control function is computed by a slow processor, the computed value may no longer be accurate.
Commercially available vapor recovery systems, such as the VaporVac.RTM. system sold by Gilbarco, Inc. of Greensboro, N.C., have a simplified control function to determine optimal vapor pump speeds. The simplified control function includes two simple sub-functions to approximate the V/L ratio. As another way to simplify the control function, U.S. Pat. No. 5,195,564 to Spalding uses a constant V/L ratio of 1.3:1.
Because a simplified control function is used, a relatively simplified processor and software can be used to solve the control function in a sufficiently short time period. But, vapor recovery systems that rely on simplified control functions are less accurate at recovering vapor. They may provide insufficient suction, letting the vapor escape to the atmosphere, or too much suction, unduly pressurizing underground pipes and tanks.
A vapor recovery system is needed that is capable of accurately controlling the rate of vapor recovery without the need of a moderate to high speed processor.